1. Field of the Invention
The present invention relates to a battery system and a battery control apparatus provided with a function of detecting a voltage value of a battery, and more particularly to a battery system and a battery control apparatus provided with a function of detecting such a fault as leads to abnormal detection of a voltage value. The present invention also relates to an electric vehicle employing such a battery system.
2. Description of Related Art
With a voltage detection apparatus that detects an analog voltage of a measurement target by use an AD (analog-to-digital) converter, breakage of the signal line across which the analog voltage is delivered, or failure of the AD converter itself, results in abnormal detection of the analog voltage (makes accurate detection of the analog voltage impossible). On the other hand, with such a voltage detection apparatus, it is often the case that the range of voltage which the measurement target analog voltage normally is in is previously known.
Accordingly, so long as the output voltage value of the AD converter is an abnormally high, or abnormally low, voltage value that the measurement target analog voltage cannot take (that is, not within the above-mentioned range of voltage), it is easy to detect some abnormality or other. There are also, however, minor faults in which the output voltage value of the AD converter is within the above-mentioned range of voltage indeed but does not satisfy the expected detection accuracy. Such faults are called in-range faults.
For example, in a case where the measurement target is a lithium-ion secondary battery, it is common to adopt, for example, about 4.0 V (volts) and about 2.0 V as the voltage values indicating overcharging and overdischarging, respectively, of the lithium-ion secondary battery. Accordingly, if the output voltage value of the AD converter is over 4.0 V or below 2.0 V, it is possible to detect overcharging or overdischarging, each a kind of abnormality. However, when the actual voltage value of the measurement target is 3.6V, if due to abnormal accuracy of the AD converter the output voltage value of the AD converter is 3.3V, the control system will recognize the voltage value of the measurement target to be 3.3V. Causing erroneous detection of 3.3 V despite its actually being 3.6 V, such abnormal accuracy is a kind of in-range fault.
FIG. 13 is an overall block diagram of a voltage detection apparatus 900 which contributes to detection of an in-range fault as discussed above. FIG. 14 is a block diagram of part of the voltage detection apparatus 900, including an equivalent circuit diagram of a voltage input switching portion 911. In the voltage detection apparatus 900, a voltage detection line 910 across which an analog voltage of a measurement target is delivered is connected to a first input terminal of a voltage input switching portion 911 comprising a multiplexer or the like, and a reference voltage having a previously known reference voltage value is fed to a second input terminal of the voltage input switching portion 911. The voltage input switching portion 911 performs selection operation such that the analog voltage of the measurement target and the reference voltage are fed to an AD converter 912 on a time-division basis. Based on the difference between the output voltage value of the AD converter 912, as observed when the reference voltage is being fed to the AD converter 912, and the reference voltage value, a comparator 913 can detect whether or not an in-range fault is present. Specifically, if the absolute value of the difference is equal to or greater than a predetermined threshold value (for example, 10 mV), the comparator 913 recognizes an in-range fault to be present.
According to another conventionally disclosed method, a measurement value obtained through conversion by an AD converter is compared with a signal having a predetermined signal level introduced from the measurement value, and if a deviation exceeding a permissible range is detected between those signal values, a faulty state within the range for detecting the measurement value is recognized.
Inconveniently, however, with the circuit configuration of the voltage detection apparatus 900, if an abnormality is present in a switch portion 920 provided within the voltage input switching portion 911, hence between the voltage detection line 910 and the AD converter 912, it cannot be detected. For example, if the on-state resistance in the switch portion 920 is abnormally high, or the leakage current in the switch portion 920 is abnormally high, although the voltage value of the measurement target is detected to be slightly lower than it truly is, since the result of AD conversion of the reference voltage is not affected by the switch portion 920, an in-range fault associated with the switch portion 920 cannot be detected.
With a view to protecting the measurement target such as a lithium-ion secondary battery from short-circuiting, and protecting the voltage detection apparatus from surges, it is also common to provide a protection circuit including a protective resistor or an anti-surge device between the voltage detection line 910 and the measurement target. With the circuit configuration of the voltage detection apparatus 900, however, a fault in such a protection circuit (for example, an abnormal resistance value in the protective resistor, or an abnormal leakage current in the anti-surge device) cannot be detected either.
On the other hand, in a case where the measurement target is a secondary battery or the like, on detection of an in-range fault it is necessary to cope with it in some way or other. No technology, however, has been proposed yet that achieves satisfactory detection of an in-range fault and that in addition can cope with one appropriately on its occurrence. Incidentally, the above-mentioned “method of detecting a faulty state within the range for detecting the measurement value” is intended for detection of the position of a mechanical member such as a gas pedal (accelerator) of an automobile, and is not technology useful in a case where the measurement target is a secondary battery or the like.